There is a desire to form an engine main body defining a cylinder and a crankcase chamber by using as small a number of component parts as possible, and support the crankshaft of the engine in a favorable manner in terms of structural integrity and ease of assembly. In a known overhead camshaft engine disclosed in JP2002-349340A, the engine main body includes a cylinder block that is integrally formed with a cylinder head, and a first bearing for supporting a first end of the crankshaft is fitted into a hole formed in the cylinder block. The lower end of the cylinder block is defined by a plane extending obliquely across the crankshaft, and a crankcase having a corresponding upper end is attached to the lower end of the cylinder block to define the crankcase chamber in cooperation with the cylinder block. The crankcase is formed with a hole, and a second bearing for supporting a second end of the crankshaft is fitted into the hole formed in the crankcase.
In this engine, the camshaft is provided in an upper part of the engine, and is actuated by the crankshaft via a timing belt which is passed around a crankshaft pulley fitted on the crankshaft and a camshaft pulley fitted on the camshaft. In the case of a four stroke engine, the camshaft pulley is required to be twice as large as the crankshaft pulley in diameter. The crankshaft pulley is positioned on a part of the crankshaft located between the second bearing and the adjoining crank web or between the second bearing and the cylinder of the engine.
This part is a load bearing part of the engine (the bending loading transmitted from the piston of the engine), and is required to have a relatively large diameter, typically a same diameter as the part of the crankshaft supported by the second bearing. Therefore, the effective diameter of the crankshaft pulley which is given as the sum of the diameter of the crankshaft and twice the thickness of the hub of the crankshaft pulley is significant. The effective diameter of the camshaft pulley is required to be twice as large as the effective diameter of the crankshaft, and the valve actuating chamber of the engine is required to be large enough to accommodate the camshaft pulley.
As a result, the mass of the crankshaft pulley and the camshaft pulley is undesirably great, and the overall height of the engine is undesirably great. Also, the assembling of the engine is undesirably complex.